1. society
2. work
3. unemployment

Outline of Chapter 11 lectures
11.1. Components of aggregate demand
11.2. Putting components together
11.3. Summary of IS curve
11.4. Movement along IS curve
11.5. Shift of IS curve
11.6. Microfoundations of IS curve
Chapter 11, part 2
Uses and microfoundations of
the IS curve
1
11.3. Summary of IS curve
2
IS curve
Yt ā b Rt
r
R t real interest rate
r marginal product of capital
b sensitivity of I t to R t
Curve passes through R t r, Y t ā
3
Real interest
rate R
4
What is the value of ā?
Suppose we’re at potential output (Y t
r
Yt
and the real interest rate is the value
predicted by the long-run model (R t r .
IS curve
ā
Short run
Output Ỹ
5
6
1
Then demands for each component of GDP
would be as follows:
C t ācYt
It
āi b Rt r Yt āiYt
G t ā g Y t , EX t ā ex Y t , IM t ā im Y t
Yt
Ct It Gt
ācYt ā iYt
āc
āi
EX t IM t
ā g Y t ā ex Y t
āg
ā ex
Yt
āc
āi
āg
ā ex
ā im Y t
So if we have Y t Y t , we would need
1 ā c ā i ā g ā ex ā im
or
ā
ā im Y t
āc
āi
āg
ā ex
ā im
1
0
ā im Y t
7
Conclusion: if the economy is in
long-run equilibrium, then ā 0.
In other words, for an economy that
is currently at potential output, the
IS curve passes through Y t 0, R t r.
We will model temporary spending
shocks as values of ā 0.
Example: consumers become more
pessimistic ā c , ā 0.
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The IS curve for an economy
currently at potential output
9
11.4. Movement along IS curve
It
Gt
EX t
Movement along IS curve
• If real interest rate
rises above MPK,
some investment
spending is
discouraged and
output would fall
below potential
Movement along IS curve:
Rt
It
Ct
10
IM t
Yt
11
12
2
11.5. Shift of IS curve
IS curve:
Yt ā b R t
• If the sensitivity of investment demand to
the interest rate were higher
r
If a temporary shock to spending
causes ā 0, then the value of Y t associated
with R t r would be greater than 0.
In other words, if ā 0, the IS
curve would shift to the right.
IS curve shifts to the right if ā
– The IS curve would be flatter
– Any change in the interest rate would be
associated with larger changes in output
13
14
IS curve shifts to the right if ā
āc
āc
āi
āg
ratio of consumption to Y t
ratio of investment to Y t when R t r
ratio of government spending to Y t
ā ex
ā im
15
1
ā
āi
āg
ā ex
ā im
ratio of exports to Y t
ratio of imports to Y t
16
IS curve shifts to the right if
ā c (consumption demand grows
faster than usual long-run trend)
ā i (investment demand ")
āg
(government spending ")
ā ex
(exports ")
ā im
(imports grow slower than
usual long-run trend)
17
18
3
Summary of IS curve
Example of using IS curve
• A change in R shows up as a movement
along the IS curve.
• Imagine that Japan enters into a
recession.
• Any other change in the parameters of the
short-run model causes the IS curve to
shift.
– The aggregate demand parameter for U.S.
exports declines.
• the IS curve shifts to the left
– thus the Japanese recession has an
international effect.
– We could shock any of the other aggregate
demand parameters.
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11.6. Microfoundations of the IS
curve
Microfoundations:
The underlying microeconomic
behavior that establishes the demands for
C, I, G, EX, and IM.
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Consumption
• People prefer a smooth path for
consumption compared to a path that
involves large movements.
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• The permanent-income hypothesis
– People will base their consumption on an
average of their income over time rather than
on their current income.
• The life-cycle model of consumption
– Suggests that consumption is based on
average lifetime income rather than on
income at any given age.
• The life-cycle model of consumption:
– Young people borrow to consume more
than their income.
– As income rises over a person’s life
• consumption rises more slowly
• individuals save more
– During retirement, individuals live off
their accumulated savings.
4
• The life-cycle/permanent-income (LC/PI)
hypothesis
– Implies that people smooth their consumption
relative to their income
– This is why we set consumption proportional
to potential output rather than actual output.
Multiplier Effects
• We can modify the consumption
equation to include a term that is
proportional to short-run output.
• With a multiplier:
– Aggregate demand shocks will increase
short-run output by more than one-for-one.
– A shock will “multiply” through the economy
and will result in a larger effect.
• If short-run output falls with a multiplier
– Consumption falls
– Which leads to short-run output falling
– Consumption falls again
– “Virtuous circle” or “vicious circle”
• Solving for the IS curve
– Will yield a similar result
– Now includes a multiplier on the aggregate
demand shock and interest rate terms:
• the multiplier is larger than one
Investment
• At the firm level, investment is
determined by the gap between the real
interest rate and MPK.
• In a simple model
– The return on capital is the MPK minus
depreciation.
• The richer framework includes:
– Corporate income taxes
– Investment tax credits
– Depreciation allowances
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• A second determinant of investment
– The firm’s cash flow
• the amount of internal resources the
company has on hand after paying its
expenses
• Agency problems
– When one party in a transaction has more
information than the other party
– It is more expensive to borrow to finance
investment because of this.
• The potential output term in the
investment equation incorporates cash
flows.
• Adverse selection
– If a firm knows it is particularly vulnerable
• it will want to borrow because if the firm
does well it can pay back the loans.
• if it fails, the firm cannot pay back the
loan but will instead declare bankruptcy.
• Moral hazard
– A firm that borrows a large sum of money
may undertake riskier investments
• if it does well, it can repay.
• if it fails, it can declare bankruptcy.
Government Purchases
• Government purchases can be
– A source of short-run fluctuation
– An instrument to reduce fluctuations
• Discretionary fiscal policy
• Captures cash flow.
• If we wish to add short-run output, it
would provide additional justification for
a multiplier.
• Transfer spending often increases when
an economy enters into a recession.
• Automatic stabilizers
– Programs where additional spending
occurs automatically to help stabilize the
economy
– Welfare programs and Medicaid are two
such stabilizer programs.
• receive additional funding when the
economy weakens
– Includes purchases of additional goods in
addition to the use of tax rates
– For example, the government can use the
investment tax credit to encourage
investment
• Fiscal policy’s impact depends on two
things:
1. The problem of timing
• discretionary changes are often put into
place with significant delay.
2. The no-free-lunch principle
• implies that higher spending today must
be paid for today or some point in the
future.
• such taxes may offset the impact of the
discretionary spending adjustment.
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• What matters for consumption today?
• The permanent-income hypothesis
says:
– What matters is the present discounted
value of your lifetime income, after taxes.
• Ricardian equivalence says:
– What matters is the present value of what
the government takes from the consumers
rather than the specific timing of the taxes.
• An increase in government purchases
financed by taxes today
– Will have a modest positive impact on the
IS curve
– Will raise output by a small amount in the
short run
• An increase in spending today financed
by taxes in the future
– Will shift the IS curve out by a moderate
amount
– Perhaps by 75 cents to $1 for each dollar
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